Text telephony encompasses transmitting text characters in-band and in real-time between distant parties using telephonic systems and equipment traditionally designed to transmit human speech. By transmitting text characters, instead of or in addition to human speech, text telephony enables deaf, hard of hearing, or speech-impaired people to communicate in a call.
Transporting text characters over a wireless communication network, however, proves challenging. A wireless communication network transports a call over a voice coded channel. A voice coded channel is a channel that is specifically optimized for transporting human speech with an acceptable call quality. A voice coded channel for instance employs lossy compression schemes optimized for transporting human speech by transporting parameters of a speech model instead of a digitized representation of the speech waveform. A voice coded channel therefore saves bandwidth in transporting human speech. In transporting text characters, however, a voice coded channel proves unreliable, with an unacceptable character error rate.
Accordingly, known approaches to addressing this problem condition the signals of all calls in the wireless communication network in a specific way, so that those signals can be reliably transported over the voice coded channel in case they represent text characters. Where the signals represent Baudot encoded text characters, for instance, these known approaches transcode the text characters to be represented with a 4 tone frequency shift keying (FSK) format which is transmitted via sophisticated features such as interleaving and convolutional coding. Represented in this 4 tone FSK format, the text characters can be more reliably transported over the voice coded channel, since the frequency of these 4 FSK tones are more compatible with the speech codec and it also augments the original TTY bit rate from 45.5 bps to 400 bps, providing the needed extra bits (per FSK) for the convolutional coding. Implementing this conditioning, however, proves expensive in terms of processing and memory requirements.